Transposable elements are one of the most unexpected discoveries of modern genetics. They were first discovered in maize and called controlling elements because of their ability to confer unstable patterns of gene expression. Maize still offers one of the choice developmental and genetic systems to investigate the genetic and molecular biology of eukaryotic transposable elements. This proposal focuses on the biology of the maize controlling element Activator (Ac), one of the best understood eukaryotic transposons. Several gaps remain in our present understanding of the mechanism of Ac transposition and chromosomal factors that contribute to the patterns of transposition during development. By combining the available genetic resources on Ac with the resolution offered by molecular biology techniques, a characterization of genetic factors that regulate transposition patterns will be continued. Ac transposition will be studied within the framework of chromosomal replication and methylation patterns using unstable alleles of complex genetic loci that precisely monitor transposition events during plant development. These studies will incorporate a mutational analysis of Ac to begin to define genetic functions associated with excision, integration, and dosage suppression activities of Ac. These investigations will generate multiple insertion alleles of Ac at defined target genes that will allow a fine structure mutational analysis of complex loci such as P and R. A metastable phenomenon associated with DNA modification of Ac that regulate transposition activity will be further investigated in the context of genomic imprinting events that are probably determined by the mode of Ac transmission during gametogenesis. Similar phenomena have been described in yeast and transgenic mice. Because transposons cause mutations and chromosome restructuring, these studies have wide-range significance for the generation of genome reorganization, allelic variation, and possibly evolution of eukaryotic organisms. Moreover, chromosomal rearrangements have been implicated in certain forms of malignancy and other types of developmental change yet we do not fully understand the mechanisms of these rearrangements.